In many applications it is advantageous to know the throughput of a liquid from a source to a load. An example is the measurement of the flow of, say, fuel oil from a tank to a furnace. Other applications include, as will be apparent hereinafter, measurements of the flow of gasoline from the tank of an automotive vehicle to the engine, measurements of the flow of diesel oil in similar applications and, more generally, the measurement of liquid flows quantitatively between a supply, pump or other source of the liquid and a drain or other load therefor.
It has been proposed heretofore to provide a flowmeter for the purposes described which comprises a measuring cylinder in which a floating piston is reciprocatable, the piston subdividing the measuring cylinder into two chambers which are alternately filled with the liquid and drained. The strokes of the piston are counted to indicate the quantity of displaced liquid, i.e. the quantity of liquid transferred between the source and the load through the measuring cylinder.
The measuring cylinder is controlled by a control cylinder or valve and within the control cylinder there is provided a movable floating control piston. The control piston is provided with retaining means for temporarily holding it in its opposite positions.
Flowmeters for measuring the throughput of a liquid from a source to a load in the above-described manner are known in various configurations including that which has been described above. An alternative form of flowmeter is the so-called dynamic flowmeter. The flowmeter which has been described in some greater detail previously is a so-called volumetric flowmeter. Volumetric flowmeters can operate as discontinuous or continuous units.
Flowmeters for liquids are employed for a variety of purposes and those which are of greatest concern at the moment are the applications for gasoline, diesel oil and heating oil.
More particularly, it is vital in many cases to provide a high-precision flowmeter to determine the instantaneous specific fuel consumption of an automotive vehicle.
Since dynamic flowmeters are density and viscosity dependent, they cannot generally be used when exact measurements are required and it is not possible to maintain either the density or viscosity of the liquid constant. It may be noted that gasoline, depending upon the composition of the fuel and temperature fluctuations, suffers a specific gravity variation of up to 10%.
The flowmeter which has been described above is of the type illustrated in German open application (Offenlegungsschrift) DT-OS 24 00 502 (see also U.S. Pat. No. 4,055,084). This unit is a volumetric flowmeter which provides reasonably accurate measurements over a wide measuring range, i.e. over throughputs which can vary widely, and has the advantage that it generates only a small pressure drop between the source and the load. Nevertheless, the measuring precision of such a flowmeter is subject to certain errors which will be elucidated below.
The flowmeter as noted comprises cylinder chambers on opposite sides of the measuring piston which are connected with the cylinder chambers of the control cylinder which is disposed between the liquid inlet and the liquid outlet (see FIGS. 1 and 2 of this publication).
The liquid entering the inlet on the flowmeter passes, depending upon the position of the control piston, into one or the other of the cylinder chambers of the measuring cylinder, displaces the measuring piston in the direction of the other cylinder and displaces the liquid volume in this other cylinder through the outlet.
When the measuring piston has reached its end position, i.e. the piston can no longer be displaced by the liquid entering the flowmeter, there develops in the first cylinder chamber of the control cylinder, a control pressure which is effective upon the control piston and releases the latter from its retaining means. The control piston is then moved and the direction of pressurization of the measuring piston and the measuring cylinder is reversed. The measuring piston is then displaced in the opposite direction.
In the operation of this unit there is an undesirable displacement of a certain amount of liquid which can be referred to as the changeover volume. This changeover volume does not pass through the cylinder chambers of the measuring cylinder in the sense that it participates in the stroke of the piston and thus is not measured by counting the strokes of the measuring piston. To take into consideration this changeover volume, a given factor must be added to the volume displacement of the measuring cylinder for each count of the stroke of the piston. With the prior-art device, however, the changeover volume is not constant and depends upon the flow velocity. As a consequence, an error is imparted to the measurement given by the system of this publication.
Mention should be made of the fact that a flowmeter for liquids is known (see "Precision Automotive Fuel, Economy Testing System" published by Fluidyne Instrumentation, Okland, California), in which four measuring cylinders are provided. The pistons within these cylinders are connected via piston rods to a common crank shaft and a single counting element is provided to count the piston strokes of the measuring piston. This flowmeter is, naturally, extremely expensive and difficult to fabricate and, because of the more complex mechanism, may not be as reliable in practice as a system using free-floating measuring pistons.
An additional disadvantage of this arrangement is the fact that it also is sensitive to the relationship between the positions of the pistons and the various flow cross sections of the measuring cylinder.